This invention is generally directed to color television receivers and particularly to so-called "color killers" in such receivers.
Conventionally, color television receivers include one or more chroma amplifiers for amplifying the chroma signal. The gain of at least one of those amplifiers is usually controlled by a burst amplitude detector. The latter device develops a d.c. voltage indicative of the size of the burst signal which forms a part of the composite television signal. On the assumption that the magnitude of the burst signal and the magnitude of the chroma signal vary together, a d.c. voltage indicative of a low level burst signal also indicates a low level chroma signal. Hence, the d.c. voltage generated by the burst detector is conventionally used to vary the gain of a chroma amplifier.
When a burst is not present on the received television signal, that is an indication that a chroma signal is also not being transmitted. Hence, color television receivers also incorporate a "color killer" to close the chroma channel, such as by turning off one of the chroma amplifiers. The latter operation inhibits noise from the chroma channel from reaching the picture tube.
A problem which arises in connection with the color killer is that it is frequently unable to distinguish between the absence of a burst signal and the presence of a very low level of burst signal. Hence, if a low level burst signal varies above and below the turn-off point of the color killer, the chroma channel will be turned off and on also, thus generating an undesirable chroma flicker. To avoid such flicker, it has been proposed that the color killer include hysteresis whereby the color channel is turned off at a first low burst level and then turned on again at a second higher burst level. Thus, if variations in the level of the burst are smaller than the difference between the first and second levels, color flicker does not occur.
The use of a hysteresis-type color killer for turning off and on a chroma amplifier presents some difficulties when the color killer and the chroma amplifier are constructed as integrated circuits, as they frequently are in modern receivers. Particularly troublesome is the additional current required and the attendant undesirable increase in power dissipation which occurs when additional circuitry is included to provide the color killer with the hysteresis effect. Hence, the desirable hysteresis effect has either not been included in receivers constructed of integrated circuits, or included only with the drawback of undesirably high power dissipation.